Glass fiber insulation is well known and has been a commercial product for many years. Generally, the insulation is made from intertwined soda lime alumina borosilicate glass fibers which are held together with a binder. The binder may be any suitable material but quite commonly is a phenol-formaldehyde resin or a urea formaldehyde resin. These binders are well known, and a spray nozzle generally applies them to the glass fibers as hot gases attenuate the fibers from a rotating device, commonly called a spinner. A conveyer collects the binder-coated fibers in the form of a blanket or batt, and heat cures the blanket to produce the final insulation. The process produces various densities by varying the conveyor speed and the thickness of the cured insulation.
It is well established that asbestos fibers when inhaled can cause significant disease in man. Though the exact mechanism responsible for the biological activity of inhaled asbestos fibers is unknown, it is widely believed that their ability to remain in the lung for extended periods of time is an important factor. Glass fibers have not been linked to disease in man. Additionally, their durability or residence time in the lung appears much less than asbestos fibers. Despite this, certain countries such as Germany have proposed regulations for the use of glass fibers in insulation products. Glass fiber compositions meeting these proposed regulations are considered to be free of suspicion and can be used for both commercial and residential installations. The problem, however, for the manufacturer is to produce glass fibers which meet the proposed regulations and standard criteria. These glasses must be fiberizable in standard wool processes, have sufficient durability in use, and have acceptable insulating properties while still meeting the compositional requirements of the proposed regulations. The proposed German regulations require that glass fibers have a numerical index (KI) greater than or equal to 40 to be considered to be free of suspicion. This index is described by the equation: EQU KI=.SIGMA.(Na.sub.2 O, K.sub.2 O, CaO, MgO, BaO, B.sub.2 O.sub.3)-2(Al.sub.2 O.sub.3),
where the values for each oxide are reported as a number corresponding to the weight percentage of that oxide in the glass composition. This requirement places severe constraints on the compositions of the glass, especially on the levels of alumina and silica in the glass composition. While one obvious attempt to meet the requirements is to reduce the level of alumina in the glass composition, such glasses tend to have poor durabilities and are difficult or impossible to fiberize by standard processes.
Recently, a number of glasses have been reported as having improved biosolubility or biodegradability. For example, Potter, U.S. Pat. No. 5,055,428, Cohen et al, U.S. Pat. No. 5,108,957, Nyssen, U.S. Pat. No. 5,332,698, and Bauer et al, U.S. Pat. No. 5,401,693, all describe glass fibers having low alumina contents and which are taught to have improved biosolubility. Additionally, published PCT applications WO 95/31411, WO 95/32925, WO 95/32926, WO 95/32927, and WO 95/35265 and published German application DE 4417230 have reported glass compositions which are said to have increased biodegradability and also meet the requirement of a KI value of .gtoreq.40. However, processing and performance problems remain. Accordingly, there is still a need in this art for a glass composition which has a KI value .gtoreq.40, while possessing acceptable processing properties such as viscosity and liquidus temperatures as well as acceptable durability in use.